Decoys launched from aircraft and airborne machines can typically be loaded in any one of many cells or canisters in a rack which can be loaded in various locations (top, bottom, sides, rear) of different aircraft or even the same aircraft. The decoys are usually stowed in the storage canister without any specific indexing.
When ejected, the body/decoy must be stable, line up with the free stream and fly predictable trajectories. These trajectories should ideally approximate the flight path of the launching aircraft and allow the decoy to radiate/receive in some desired sectors, usually rear and/or front and particularly in the rear sector, below the horizontal.
Most decoys follow unpowered quasi-ballistic trajectories at essentially zero lift. Then, they quickly sink away from the aircraft path with increasing vertical velocities which facilitate discrimination. Further, the attitude of a stable non-lifting body closely matches the increasingly steep slopes of the ballistic trajectory. Then, the center line of an antenna beam is tilted upwards towards the vertical, reducing its effectiveness. Practical effectiveness is often terminated when the lower edge of the beam reaches the horizontal.
All these factors, and many other important ones, e.g. vertical and longitudinal separation from the launching aircraft, etc., are directly related to the trajectories. Obvious improvements can be achieved with lifting glide trajectories.
In the steady glide, vertical sink velocities and glide path angles become quasi-constant. Both the flatter glide path and the positive angle of attack of the body improve the downward orientation of the rear beam. At high dynamic pressures, when lift exceeds weight, the decoy can even climb initially, further increasing its useful lifetime.
This is illustrated in FIG. la which shows three trajectories of the same configuration trimmed at different conditions:
trajectory 1, trimmed at .alpha.=O.sub.1 zero lift, ballistic trajectory PA1 trajectory 2, trimmed at .alpha..perspectiveto.6.degree.-8.degree. intermediate lift/drag.apprxeq.1 PA1 trajectory 3, trimmed at .alpha..apprxeq.20.degree. maximum lift/drag ratio.perspectiveto.2 PA1 line up in the free stream direction PA1 roll to the desired attitude PA1 stabilize at the desired angle of attack with null moments about all three axes. PA1 The "pendulum" rolling moments are very small, a few pound inches at most. In steady flight, they must be augmented by much larger stabilizing aerodynamic rolling and damping moments. PA1 The aerodynamic rolling moments may be much larger than the "pendulum" rolling moments at some dynamic pressure level. Over the range of conditions and throughout the roll, the sum of the "pendulum" and aerodynamic rolling moments must remain favorable. PA1 To avoid tumbling the empennages must also maintain adequate levels of pitch and yaw stability over a wide range of angles of attack. PA1 Vertically asymmetrical configurations, with the empennages deployed in the upper rear quadrant, to locate the neutral point above as well as behind the center of gravity. PA1 A laterally symmetrical empennage layout. Each side provides both pitch and yaw forces and moments as well as a positive dihedral effect stabilizing the configuration about all three axes. PA1 To minimize inertial cross couplings, the orientation of the resultant aerodynamic force on each empennage should preferably be aimed toward the roll axis of inertia.
Equally spaced time intervals T.sub.1, T.sub.2, T.sub.3, T.sub.4, etc., identify decoy positions at comparable times along each trajectory.
Assuming 90.degree. beam angles, as sketched, the effectiveness of the decoy along trajectory 1 is nearly lost at time T.sub.2. The flight path angle is close to 45.degree. and the rear beam is essentially above the horizontal.
Trajectory 2 climbs above the initial altitude h.sub.o and still shows some effectiveness at time T.sub.4. Trajectory 3 is effective throughout and beyond T.sub.6 into the stable glide portion of the trajectory.
As shown in FIG. 1b, a given decoy configuration launched at either high or low dynamic pressures will eventually stabilize in equilibrium glide at very similar values of flight path angle, body angle of attack, and beam orientation. Effectiveness can be maintained over a wide range of operating conditions.
Increasing the lift-to-drag ratio flattens the flight path. Flying at substantial lift-to-drag ratios also means substantial levels of body angle of attack, particularly when dealing with aerodynamically unrefined decoy bodies with relatively large drags at zero lift. Then, the beam center lines can remain essentially horizontal, not only in glide, but even throughout the trajectory.
High levels of effectiveness can be maintained over a wide range of dynamic pressure until either vertical separation (minimized by the lift forces) or longitudinal separation or some combination of parameters reduces effectiveness below desired levels.
The advantages of lifting trajectories are evident, but they assume not only lift but indexing of the lift forces upwards, against gravity. Achieving this desired orientation with a randomly indexed body ejected in random orientations becomes a major goal of the invention.